The N-terminal region of the adenovirus transforming protein E1A binds to several host proteins whose functions are critical for the cell cycle control. These include p400, p300 and the highly related CBP, Retinoblastoma protein (Rb), and two Rb related proteins p107 and p130. E1A interactions with these host proteins result in profound alterations in cellular growth control, p300/CBP is a coactivator and a histone acetyl transferase that links chromatin remodeling with transcription. The focus of this application is to investigate the roles played by p300 in cell cycle control and to investigate how E1A binding to p300 deregulates the cell cycle related p300 functions. Recently, this laboratory has shown that depletion of p300 in quiescent cells leads to premature G1 exit and upregulation of c-Myc. Conversely, overexpression of p300 in quiescent cells leads to downregulation of c-Myc followed by inhibition of S phase entry. Thus, p300 provides an important function in G1 control of the cell cycle by negatively regulating Myc, and preventing a premature G1 exit. These results provide a molecular basis for the previously documented need for viral oncoproteins such as E1A and SV40 large-T to inactivate the p300 functions during cell transformation. Here, studies are proposed to investigate the mechanism by which p300 negatively regulates c-Myc at the chromatin level. The specific aims include the identification of the domains and activities of p300 that are critical for the negative regulation of Myc, the Myc promoter specific transcription factors and chromatin remodeling proteins that interact with p300 during this negative regulation, and the mechanism of their interactions that results in the repression of the Myc promoter. Finally, studies are proposed to investigate the mechanism by which EIA inactivates p300 functions that leads to Myc upregulation, and how E1A binding to p300 contributes to the G1 exit. p300/CBP is a key regulator of cell proliferation with tumor suppression properties. Therefore, understanding the functions of this protein in cell cycle control, and the mechanism by which viral oncogene products interfere with its functions will be valuable in understanding the p300 mediated growth regulatory pathways.